Unbalances between the electrical power fed into an electrical grid and the electrical power withdrawn from it lead to fluctuations of the grid frequency. If the electrical power generation drops below the power consumption from an electrical grid, e.g. due to a power plant failure or disconnection, the grid frequency drops. Conversely, if the power consumption falls below the amount of electrical power generation, the grid frequency increases. In order to compensate for such frequency fluctuations, there are power generating stations which are arranged to continuously vary their active electrical power output until the unbalance has been eliminated. This electrical output power variation is called “primary power control”. Grid operators specify the primary power control requirements in so-called Grid Codes as, for example, the Grid Code 2006 by E.ON Netz GmbH, English version, published by E.ON Netz GmbH, downloadable at http://www.eon-netz.com/pages/ene_de/Veroeffentlichungen/Netzanschluss/Netzanschlussregeln/ENENAR HS2006eng.pdf.
Currently, wind turbines generally do not contribute to primary power control, mainly because the power source “wind” is not controllable. However, with the increasing proportion of wind energy plants in the overall electrical power production, a contribution of wind turbines to primary power control is desired.
Also, mechanical rotating parts of the energy conversion system of modern variable speed wind turbines are not electrically coupled to the electricity network, thus the wind turbine is mechanically decoupled from the grid, differently to conventional fixed speed synchronous generators. In this way, modern wind turbines do not have an inherent contribution to the grid stability when a grid event is experienced, such as sudden imbalances between total generation and consumption in the network (due to generator trip or load trip), differently from conventional fixed speed synchronous generators. Such wind turbines are thus not contributing with the grid rotating inertia. With the increasing proportion of wind energy plants in the overall electrical power production, the number of fixed speed synchronous generators is decreasing, thus loosing the inherent capability of generation mix to support grid stability when a grid event is experienced such as sudden imbalances between total generation and grid consumption. The total grid inertia is decreased, deteriorating the grid frequency stability. A contribution of wind turbines with fast controlled active power modulation for grid stability is desired.
It is known, for example from DE 100 22 974 A1, that wind turbines can react to grid frequency increases (i.e. less power is consumed from the electrical grid than is fed into it) by decreasing their output power. It is, however, hard to respond to frequency decreases (i.e. more power is consumed from the electrical grid than is fed into it) because that means increasing the active electrical power production without having more wind energy available. Two different approaches are known to address this issue:
Firstly, two papers by Harald Weber et al. from Rostock University (“Netzregelverhalten von Windkraftanlagen”, published at the conference 6th GMA/ETG-Fachtagung “Sichere und zuverlassige Systemführung von Kraftwerk und Netz im Zeichen der Deregulierung”, held from 21 to 22 May 2003 in Munich, downloadable at www.e-technik.unirostock.de/ee/download/publications_EEV/uni_hro_publ35_WKA_2003.pdf; “Primärregelung mit Windkraftanlagen”, published at the ETG-Workshop “Neue dezentrale Versorgungsstrukturen”, held from 19 to 20 Feb. 2003 in Frankfurt/Main, downloadable at www.e-technik.unirostock.de/ee/download/publications_EEV/uni_hro_publ33_etg_frankfurt_2003.pdf; both documents are hereinafter referred to as the “Rostock papers”) recommend the operation of a wind turbine at a suboptimal working point (e.g. at a higher than optimal rotational rotor speed, at a given wind speed) in order to have power reserves available which can be additionally output in the case of a frequency drop (e.g. by then lowering the rotational rotor speed to the optimal speed, at a given wind speed). By this, the additional electrical output power can be fed into the grid over an indefinite time.
According to the second approach, which is for example outlined in WO 2005/025026 A1, the kinetic energy stored in a wind turbine's rotor is identified as a power reserve that can be transformed into electrical power and additionally injected into the grid, however, only over a short time period. By using kinetic rotor energy, it is also possible to compensate periodic frequency oscillations, by periodically de-accelerating and acceleration the rotor, in synchronisation with the frequency oscillation.
A similar concept for short-time power input at the cost of kinetic rotor energy is provided by the article “Temporary Primary Frequency Control Support by Variable Speed Wind Turbines—Potential and Applications” by Ullah et al. (published by IEEE in “IEEE Transactions on Power Systems, Vol. 23, No. 2” in May 2008, pages 601 to 612, downloadable at ieeexplore.ieee.org/iel5/59/4494587/04480153.pdf; hereinafter referred to as “ULLAH”).
These proposals for using kinetic energy from the rotor to temporarily output additional electric power are, however, not yet matured as said documents are not concerned with wind turbine controlling after a non-periodic additional electrical power output has ended. The present invention provides a refined approach for a fast active power variation for grid stability and primary power control contribution by wind turbines.